Question

What is the possible arrangement of the isomers of molecular formula C4H9Cl in order of decreasing rate of reaction with sodium iodide in acetone?

Answer 1

The order is butyl > isobutyl > sec-butyl > tert-butyl.

Explanation:

The reaction is an `"S"_text(N)2` displacement reaction:

`"I"^"-" + "R-Cl" → "I-R" + "Cl"^"-"`

Thus, a major factor affecting relative reaction rates is steric hindrance to attack by the nucleophile `"I"^"-"`.

Let's examine ball-and-stick and space-filling models of the isomeric chlorides.

In the space-filling models, I have marked the backside of the carbon bearing the leaving group with a yellow dot.

Butyl chloride

This is a primary alkyl halide.

It has the least steric hindrance to approach by the nucleophile, so it should have the fastest rate.

Isobutyl chloride

This is also a primary alkyl halide.

However, the branched isopropyl group provides more steric hindrance than the linear propyl group in butyl chloride.

This is the next fastest reacting chloride.

sec-Butyl chloride

The methyl and ethyl groups severely hinder approach by the nucleophile. This is the third fastest reacting chloride.

tert-Butyl chloride

The carbon bearing the `"Cl"` atom is almost completely shielded by the three methyl groups. This is the slowest reacting of the four chlorides.